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The Chemical Educator

ISSN: 1430-4171 (electronic version)

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Abstract Volume 18 (2013) pp 080-084
DOI 10.1007/s00897132470a

Determination of Dissociation Constants of Weak Acids and Bases Using Indicators

Saptarshi Ghosh and Nitin Chattopadhyay*

Department of Chemistry, Jadavpur University, Jadavpur, Kolkata – 700 032, India, nitin.chattopadhyay@yahoo.com
Received December 20, 2012. Accepted February 23, 2013.

Published: 12 April 2013

Abstract. The conventional use of acid-base indicator is extended for the determination of dissociation constants of weak acids and bases through the measurement of the solution pH. In aqueous solution, depending on the pH of the solution, the indicator exhibits dual absorption bands characteristic to the two conjugate forms of the molecule. The ratiometric absorbance of an indicator at the two absorption bands is taken as a function of the solution pH to prepare a calibration plot. To determine the dissociation constant of a weak acid or base, one has to simply make a solution of it with known concentration, add the proper indicator, take the absorption spectrum and hence the ratiometric absorbance. Interpolation of this value on the calibration plot gives directly the pH of the solution from where the pK value can be determined from the known concentration. To encompass a wide range of pHs, a number of indicators covering different pH ranges are used. The strategy, based on the ratiometric principle, overcomes many of the instrumental as well as experimental artifacts. Once the literature is enriched with such calibration plots with different indicators for various pH ranges, a student will not need a pH meter for the purpose.s

Key Words: Laboratories and Demonstrations; general chemistry; acid-base chemistry; dissociation constants of weak acids and bases; strategic study; ratiometric absorption

(*) Corresponding author. (E-mail: nitin.chattopadhyay@yahoo.com )

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